1. Field of the Invention
The present invention relates to a magnetic recording element that is used for thermally-assisted magnetic recording in which information is recorded while decreasing the coercive force of a magnetic recording medium using near-field light irradiated onto the magnetic recording medium.
2. Description of the Related Art
In recent years, in association with the high recording density of magnetic recording devices such as magnetic disk devices, there are demands for performance improvements of a thin film magnetic head and a magnetic recording medium. A composite-type thin film magnetic head, in which a reproducing head having a magneto resistive effect element (MR element) for reading and a recording head having an inductive-type electromagnetic transducer (a magnetic recording element) for writing are laminated on a substrate, has widely been used for the thin film magnetic head. In magnetic disk devices, the thin film magnetic head is disposed on a slider that flies just above a surface of the magnetic recording medium.
The magnetic recording medium is a discontinuous medium on which magnetic microparticles gather. Each of the microparticles has a single magnetic domain structure. In the magnetic recording medium, one recording bit is configured with a plurality of magnetic microparticles. In order to increase the recording density, the asperity of a boundary of adjacent recording bits needs to be small. For this, the size of the magnetic microparticles needs to be decreased. However, when the size of the magnetic microparticles is decreased, the volume of the magnetic microparticles decreases. Accordingly, thermal stability of the magnetization of the magnetic microparticles also decreases. In order to solve this problem, increasing the anisotropic energy of the magnetic microparticles is effective. However, when the anisotropic energy of the magnetic microparticles is increased, the coercive force of the magnetic recording medium is also increased. As a result, it becomes difficult to record information using a conventional magnetic recording element. The conventional magnetic recording element has such a drawback, and this is a large obstacle to achieving an increase in the recording density.
As a method to solve this problem, a so-called thermally-assisted magnetic recording method is proposed. In this method, a magnetic recording medium having a large coercive force is utilized. The magnetic field and heat are simultaneously added to a part of the magnetic recording medium where information is recorded when the information is recorded. Using this method, the information is recorded in a state where the temperature of the information record part is increased and the coercive force is decreased.
For a thermally-assisted magnetic recording, a method in which a laser light source is used to heat the magnetic recording medium is common. Such a method, is really two methods: one method is to heat the magnetic recording medium by guiding laser light to a recording unit via a waveguide, etc. (a direct heating type); and the other method is to heat the magnetic recording medium by converting the laser light to near-field light (a near-field light heating type). The near-field light is a type of so-called electromagnetic field, which is formed around a substance. Ordinary light cannot be focused to a region that is smaller than its wavelength due to diffraction limitations. However, when light having an identical wavelength is irradiated onto a microstructure, near-field light that depends on the scale of the microstructure is generated, enabling the light to be focused onto a minimal region, such as a region only tens of nm in size.
In U.S. Patent Application Publication No. 2008/205202, another configuration is disclosed in which a near-field generator is disposed in a front part of a core of a waveguide through which light from a light emission element (a laser diode) propagates.
In U.S. Patent Application Publication No. 2008/151431, a configuration is disclosed in which a near-field generator plate and a near-field scatter plate are disposed in a front part of a waveguide in which the light enters and propagates. The near-field generator plate has a sharp end part on one end. The near-field scatter plate is arranged along an edge part that is on the opposite side of the sharp end part of the near-field generator plate.
In Japanese Laid-Open Patent Publication No. 2009-070554, a configuration is disclosed in which a low refractive index part made of SiO2 is disposed between a near-field generator and a core of a waveguide in which light enters and propagates. A refractive index of SiO2 that configures the low refractive index part is smaller than that of Ta2O5 that configures the core.
In U.S. Patent Application Publication No. 2008/239541, a configuration is disclosed, in which a second core is disposed between a near-field generator and a first core of a waveguide into which light enters and propagates. It discloses that the second core is preferably configured by alternatively laminating two types of materials having different refractive indices.
A so-called plasmon antenna is used to generate the near-field light in a conventional concrete method, in which a metal, referred to as a near-field light probe, is used for generating the near-field light by light-excited plasmon.
Direct irradiation of light generates the near-field light in the plasmon antenna. However, with this method, a conversion efficiency for converting irradiated light into the near-field light is low. Most of the energy of the irradiated light on the plasmon antenna reflects off the surface of the plasmon antenna or is converted into thermal energy. The size of the plasmon antenna is set to be the wavelength of the light or less. Accordingly, the volume of the plasmon antenna is small. Therefore, the temperature of the plasmon antenna significantly increases according to the above-described generation of heat.
Due to the temperature increase, the volume of the plasmon antenna expands, and the plasmon antenna protrudes from an air bearing surface that is a surface facing the magnetic recording medium. Accordingly, the distance between an edge part of the air bearing surface of the MR element and the magnetic recording medium increases. As a result, it becomes difficult to read servo signals recorded on the magnetic recording medium during the recording process. Moreover, when the heat generation is large, the plasmon antenna may melt.
Currently, a technology is proposed that does not directly irradiate light onto the plasmon antenna. For example, U.S. Pat. No. 7,330,404 discloses a technology in which light propagating through a waveguide, such as an optical fiber, is coupled in a surface plasmon mode via a buffer portion to a plasmon generator, so that the surface plasmon is excited in the plasmon generator. In this technology, the light is not directly irradiated onto the plasmon antenna. The plasmon generator includes a near-field generator that is positioned on the air bearing surface and that generates the near-field light. At the interface between the waveguide and the buffer portion, the light propagating through the waveguide is completely reflected. However, at the same time, light is generated that penetrates into the buffer portion, which is referred to as evanescent light. The evanescent light and a collective oscillation of charges in the plasmon generator are coupled, and then the surface plasmon is excited in the plasmon generator. The excited surface plasmons propagate to the edge of near-field-generator along the plasmon generator, so that near-field light is generated in the near-field generator. According to this technology, since the light propagating through the waveguide is not directly irradiated to the plasmon generator, the excessive temperature increase of the plasmon generator can be prevented.
In a thermally-assisted recording, which performs a recording while heating a predetermined position of the magnetic recording medium, temperatures of both the magnetic recording medium and a magnetic recording element itself are increased (for example, to approximately 200-300° C.). This temperature increase is caused due to laser light that is directly guided to the vicinity of the air bearing surface by being propagating through the waveguide, and by a loss that occurs when the laser light is converted to the near-field light.
Generally, a material of the waveguide in the magnetic recording element is TaOx, AlOx, AlNx, SiOx, SiNx, SiON, MgFx, Si, or the like. Of these materials, the materials having a high refractive index are used for the core, and the materials having a low refractive index are used for a cladding. Each element configuring these materials has a characteristic of a covalent bond and has a dangling bond on the surface. The dangling bond is active. Accordingly, an interface between such materials is significantly active. This is obvious from that, for example, Lewis acid sites and/or Bronsted acid sites are generated in solid solutions of oxides and function as a catalyser.
When the temperature of the magnetic recording element itself increases, as described above, high pressure (for example, approximately 10 atmospheric pressure) applied to the air bearing surface at the time of the slider flying, and water vapor in atmosphere applied to the interface of different materials under a state having such chemical active sites, may cause an alteration of the material and/or a deformation of the thermally-assisted magnetic recording head to occur.
Specifically, in an experiment by the applicant, when a thermally-assisted recording was executed using a magnetic recording element that includes a waveguide having a core made of TaOx and a cladding made of AlOx, it is confirmed that Al is bonded to hydroxyl (AlOH) and adheres on the magnetic recording medium. Also, Ta may be melted and altered by touching the water vapor under high temperature and high pressure.
Also, when the heat expansion ratios of the materials contacting each other at the interface are different, huge stress may cause a gap to be formed between the materials due to the difference of the heat expansion at the time of heating, which may result in the deformation of the magnetic recording element.